Use pipe between ceph backup diff export/import

We now use a piped transfer between the rbd export-diff
and import-diff for incremental backups/restores as
opposed to holding the entire diff in memory.

Change-Id: I33476d9b3934781413af5cd2867a11d825a5d78e
Fixes: bug 1244464
Fixes typo in method name _notify_voloume_type_error

Closes-Bug: #1246228

Change-Id: I7b408f2c4085ba5af5fc4b539000e5675bdc5a76
clean up numeric expressions in test

Replace numeric expressions with constants in tests
to make code more readable.

Change-Id: I42469cdbe3cafd36f8d9855c8284f885ffa7f465
Merge "Fixes typo in method name _notify_voloume_type_error"
Merge "Use pipe between ceph backup diff export/import"